This study employed proxy data to investigate the phase relationship between the North Atlantic deep-level temperature and the Qinghai-Tibet Plateau(TP)surface air temperature(TP temperature)and its evolution at the millennial scale since the Last Interglaciation.The results indicate the alternation of in-phase and anti-phase relationships since the Last Interglaciation,with the in-phase relationships showing a shorter duration than the anti-phase relationships.Alternations between the in-phase and antiphase relationships occurred more frequently during the Last Interglaciation than during the Last Glaciation.The phase relationship between the North Atlantic deep-level temperature and the TP temperature was broadly illustrated by that between the North Atlantic temperature(based on oxygen isotope data from the Greenland ice core)and TP temperature.Furthermore,the North Atlantic deep-level temperature and the TP temperature may be connected through the North Atlantic sea surface temperature.
The ring-width chronology of a Juniperus przewalskii tree from the middle of the Qilian Mountain was constructed to estimate the annual precipitation (from previous August to current July) since AD 1480.The reconstruction showed four major alternations of drying and wetting over the past 521 years.The rainy 16th century was followed by persistent drought in the 17th century.Moreover,relatively wet conditions persisted from the 18th to the beginning of 20th century until the recurrence of a drought during the 1920s and 1930s.Based on the Empirical Mode Decomposition method,eight Intrinsic Mode Functions (IMFs) were extracted,each representing unique fluctuations of the reconstructed precipitation in the time-frequency domain.The high amplitudes of IMFs on different timescales were often consistent with the high amount of precipitation,and vice versa.The IMF of the lowest frequency indicated that the precipitation has undergone a slow increasing trend over the past 521 years.The 2-3 year and 5-8 year time-scales reflected the characteristics of inter-annual variability in precipitation relevant to regional atmospheric circulation and the El Ni?o-Southern Oscillation (ENSO),respectively.The 10-13 year scale of IMF may be associated with changing solar activity.Specifically,an amalgamation of previous and present data showed that droughts were likely to be a historically persistent feature of the Earth's climate,whereas the probability of intensified rainfall events seemed to increase during the course of the 19th and 20th centuries.These changing characteristics in precipitation indicate an unprecedented alteration of the hydrological cycle,with unknown future amplitude.Our reconstruction complements existing information on past precipitation changes in the Qilian Mountain,and provides additional low-frequency information not previously available.
